Parts of the mesh are appearing pitch black (only) when I upload it with animation.

My scene contains one fairly optimized mesh (by polycount) with two animation takes and few 2k textures (although I'm pretty sure that textures are not causing any trouble as the problem persists even when I upload mesh without any texture). When exporting .fbx with animation I did everything sketchfab staff suggested, I selected only objects that I need + first joint in the hierarchy..

I did some experimenting viewing the scene with a few different browsers on different computers and found out that browsers with disabled webGL2 aren't able to preview the scene properly. It's confusing because I saw some pretty heavy scenes on the website that are rendering normally (on the browsers with disabled webGL2) contrary to my scene where there's not too much going on but still problem exists. I mean, should I even treat this as a problem? Why is webGL2 necessary for my scene? What is so special about it?

One more thing that might be interesting is that when I'm viewing the mesh in matcap or shadeless mode I'm able to see some textures (normal and diffuse).

I'm probably overlooking something very obvious... Can you help me find out what is causing the trouble in my scene?

Thank you nomadking for your quick reply. I've already read the discussion you sent me.

Yap, I probably have pushed a little too much with the number of facial joints, I have total of 54 joints for the whole model.

I'm just reading Unity blog post on the topic of optimizing bones.. It's kinda too technical for me at this point but for what I understood it might also have something to do with vertices that are getting influenced by more than 4 joints...?

It could indeed be the number of bones influencing each vertex... although your card is a bit above the spec of the integrated one the other user was having problems with, so the issue might end up being entirely different!

Probably best to avoid doing much with it until @james has been in and dropped some technical knowledge on us both

@lazaridjoski we limit the number of bones influence to 4 by vertex so this is not the culprit here (note that it could change the way the animation appears on Sketchfab but this should not be too noticeable if at all).As for webgl2, our job is to make scenes hopefully viewable everywhere so webgl2 capable devices is not a requirement.Then bones are indeed costly. Maybe @stephomi can shed some more light here too?

Sketchfab webgl2 support is only enabled for staffs, so it has nothing to do with webgl2.As for bones, we split the geometry if needed at runtime, so the number of bones shouldn't be an issue.(it's possible there is an issue on that subject but I'll need a screenshot of http://webglreport.com/?v=1 on faulty browsers to make sure of that)

I did some experimenting viewing the scene with a few different browsers

What browsers exactly? You can also check in the console if there are errors.

Too many bones is not an issue because we split the geometry to reduce the number of bones per geometry.So it should work everywhere.Having the "Max Vertex Uniform Vectors" value from http://webglreport.com/ would be useful.This number is the value used to determine in how many part we'll split the geometry.